The present invention relates to a magnetic disk apparatus for reading medium information by an MR head and, more particularly, to a magnetic disk apparatus which can properly recover a sync byte pattern at the head of a sector when such a sync byte pattern is lost by a thermal asperity ie. therefor collision heat generation (friction heat) occurring when the MR head comes into contact with a medium.
In recent years, in association with an increase in capacity of a magnetic disk apparatus as an external storage device of a computer, a magnetic head of a high performance is requested. As a magnetic head satisfying the request, attention is paid to a magnetic disk apparatus having what is called an MR head using a magneto-resistive element which can obtain a high reproduction output without depending on a peripheral velocity of a recording medium. In the magnetic disk apparatus using such an MR head as a read head, however, when the MR head collides with a physical convex or concave portion due to an extremely slight dent, a deformation, or the like on the surface of the medium which is rotating, a temperature of the MR head rises instantaneously by a friction heat. When the temperature of the MR head rapidly rises by the contact with the medium as mentioned above, a base line of a read signal is shifted, and a read error which cannot be recovered occurs. This state is seemingly the same as when a medium defect exists. The phenomenon in which the read error occurs by the collision heat generation of the MR head with the medium is usually called a thermal asperity. That is, when a state in which a sync byte pattern cannot be read as a result of thermal asperity of the MR head in a sync byte region in a read sector on a medium track, sector data cannot be demodulated at all. In this case, although the reading operation is executed again, since the defect caused by the thermal asperity of the MR head is a kind of physical defects, the same reading impossible state repetitively occurs in the same sync byte region and an unrecoverable read error is caused. For a high density recording of the medium, it is necessary to reduce a floating height of the MR head and this results in a factor of an increase in number of times of occurrence of the defect due to the thermal asperity of the MR head. As a rotational speed of the medium increases, when the thermal asperity of the MR head occurs, the shift of the base line of the read signal further increases. Further, every possible tests have been performed to the magnetic disk apparatus at a factory stage. The defect caused by the thermal asperity of the MR head is, however, a problem occurring during the use by the user. Further, since there is a tendency such that the defect grows while the user is using the apparatus, there is a fear that the performance of the magnetic disk apparatus remarkably deteriorates.